I spoke to one of the admissions committee members at the end of August about how people are selected and he said that if 5 committee members approve your acceptance you'll get in without getting on the alternate list. If 4 committee members approve, then you'll be an alternate, but there is a good chance you'll get in later in the season. So, I think the main rank is based on the number of committee members there are giving your file the okay. Then, if I remember correctly, there is a GPA/MCAT rank, with ties going to USC alumni. Finally, I think if there are any other sort of disagreements about whether you will get accepted the last word goes to your interviewers.When I spoke with admissions recently they told me the alternate list is unranked as of now, but that at the very end of the season (May 15 or something) they look at everyone on the alternate list again and make admissions decisions...so I assume that they do have some way of ranking us then (I was confused about the unranked thing)
I tried to recall as much as I could from memory and I had some notes that I wrote down as well, but I may have missed some piece of the details or remembered something incorrectly.